This invention relates to a new and improved method and system for gain selection in the amplification of electrical signals.
In U.S. Pat. No. 3,302,166 to Joseph Zemanek, Jr., there is disclosed an acoustic velocity logging system wherein a downhole tool comprises a plurality of transducers including a transmitter and multiple receivers. The receivers are spaced at different distances from the transmitter such that an acoustic pulse from the transmitter arrives at the receivers by way of different travel paths through the formation surrounding the borehole. Upon generation of an acoustic pulse by the transmitter, a transmitter trigger pulse is transmitted uphole by way of a conductor in the logging cable. The analog signals produced by the receivers in response to the arrival of the acoustic pulse at the receivers are also transmitted uphole by way of conductors within the logging cable.
Receiver signals from borehole logging systems such as described in the aforementioned U.S. patent to Zemanek, Jr. have typically been amplified and transmitted over several miles of logging cable in analog form. In U.S. Pat. No. 3,518,679 to Baldwin and Glover, there is disclosed an acoustic velocity logging tool having an analog amplification system in which a downhole switching means is employed to change the gain of the downhole analog amplifier through which the receiver signals are passed to the logging cable. Such switching means employs a bank of resistors to which the receiver signals are applied. A rotary switch is controllable from uphole so as to select the output of one of the resistors in such bank for application to the input of the analog amplifier. In this manner, gain selection for the analog amplification system is controlled through attenuation of the receiver signals prior to amplification by the downhole analog amplifier. The amplified analog receiver signals are then transmitted uphole over the logging cable in real time.